Texas Instruments 99/4A - Page 21

OSCILLATOR CIRCUIT
GENERAL DESCRIPTION
The TMS 9900 microprocessor , as used in the TI 99/4, uses four non
overlapping clock signals generated by a 74LS362 clock generator/
driver. These -Four clock signals (QI, 02, 03 and 04) are available
at TTL level. A timing diagram for the clock signals is given in
figure 35, p.114. As can be seen from the internal schematics of
the 74LS3S2 figure 35, there is also a D type latch available.
This latch, clocked by 03, is in the reset circuitry.
HARDWARE DESCRIPTION (SEE FIGURE 2, P.14.)
POWER CONNECTIONS
Power connection points are pin 10 for ground, pin
and pin 20 for the +5 volt supply. L603, L604,
C607 are used for decoup 1 ing * of noise generated by
13 for the + 12v
C604, C605 and
the 74LS362.
POWER UP
Pin 17, connected to the 5 volt power supply by serieg resestor
R604 enables the clock outputs.
RESONATOR FART
A third overtone, 48MHz crystal , connected between pin IS and 19
determines the clock frequency. The tank circuit with L602 and
L603 connected between' FINS 1 and 2 selects the third overtone.
Some units may use a 12 MHZ crystal with tank components to select
the primary frequency.
CLOCK SIGNALS
Fin 8, 9, 11 and 12 are the four 12 volt clock outputs for ths TMS
9900. The four series resistors R601, R602 and R603 minimize over
and undershoot. The inverted clock signals on pin 6, 7, 14 and 15
are used internally in the 99/4 for timing purposes.
RESET
Reset is accomplished during power-up and when a solid state soft 
ware command module is inserted. C606 will be slowly charged by
resistor R605 after applying power to the console. Thus for a
short period the D input of pin 5 of the 74LS362 is' low. The
74LS362 clocks this zero binary value to the reset out of pin 4.
This signal resets the TMS 9900 until a binary 1 value is sensed
on pin 5. When the system is switched off, R606 discharges C606,
thus enabling a new reset. When a solid state software command
module is inserted, the reset line is connected to -5 volt. C606
is then partly discharged by C506, thus dropping the voltage at
pin 5 for a short time to the binary low level, generating a
reset. When the solid state software command module is removed,
resistor R514 will discharge C506 to enable a new reset.
13